The present invention relates generally to a method for activating the electron emission surface of a field emission display. More particularly, the present invention relates to a method for activating electron emission surface by employing a spray coating technology. By using compressed air, the solution is homogeneously spray coated on each pixel of the cathode structure. The solution forms a film after being dried, and the dried coating film is peeled off using a peeling apparatus, so as to activate the electron emission surface.
Conventional triode field emission display includes an anode structure and a cathode structure. There is a spacer disposed between the anode structure and the cathode structure, thereby providing a space and a support for the vacuum region between the anode structure and the cathode structure. The anode structure includes an anode substrate, an anode conducting layer, and a phosphorus layer, while the cathode structure includes a cathode substrate, a cathode conducting layer, an electron emission layer, a dielectric layer and a gate layer. The gate layer is provided a voltage difference to induce the emission of electrons from the electron emission layer. The conducting layer of the cathode structure provides a high voltage to accelerate the electron beam, such that the electron beam can have enough kinetic energy to impinge and excite the phosphorous layer on the anode structure, thereby emitting light. Accordingly, in order to maintain the movement of electrons in the field emission display, a vacuum apparatus is required to keep the vacuum degree of the display being below 10−5 torr. Therefore, the electrons can have appropriate mean free paths. Meanwhile, the pollution and toxication of the electron emission source and the phosphorous layer should be prevented from happening. Furthermore, in order for the electrons to accumulate enough energy to impinge the phosphorous powder, a space is required between the two substrates. Consequently, the electrons can be accelerated to impinge the phosphorous layer, thereby exciting the phosphorous layer and emitting light therefrom.
The electron emission layer is composed of carbon nanotubes. Since carbon nanotubes, discovered by Iijima in 1991 (Nature, 354, 56 (1991)), comprises very good electronic properties that can be used to build a variety of devices. The carbon nanotubes also has a very large aspect ratio, mostly larger than 500, and a very high rigidity of Young moduli larger than 1000 GPn. In addition, the tips or defects of the carbon nanotubes are of atomic scale. The properties described above are considered an ideal material for building electron field emitter, such as an electron emission source of a cathode structure of a field emission display. Since the carbon nanotubes comprise the physical properties described above, a variety of manufacturing process can be developed, e.g. screen printing, or thin film processing.
However, the art of manufacturing the cathode structure employs carbon nanotubes as a electron emission material, which is fabricated on the cathode conducting layer. The manufacturing process can employ chemical vapor deposition process, or any kind of process that can pattern the photosensitive carbon nanotube solution on any pixel of the cathode conducting layer. Moreover, the cathode structure can also be manufactured by coating the carbon nanotubes solution while incorporating with a mask, or depositing the carbon nanotubes on the cathode conducting layer by an electrophoresis method. Nonetheless, the cathode structure needs to be sintered in a 500° C. oven, so as to remove the residual solvent left on the cathode structure and to enhance the adhesion of the carbon nanotubes affixed on the cathode conducting layer.
However, the manufacture process of the cathode structure is not completed yet. In general, a so-called surface activation process is needed after the high temperature sintering process. Since the high temperature sintering process still can not remove other non-nanotube carbon bulks, other non-crystalline carbon nanotube, other bulky carbon balls, or other organic materials formed during the high temperature sintering process on the electron emission surface. The impurities described above can affect the electron production rate of the carbon nanotube electron emitter. Therefore, the surface activation process described above is deemed necessary, so as to remove or transform the above impurities, thereby enhancing the electron production rate of the electron emitter.
One conventional method is disclosed in the Taiwanese publication no. 480537, entitled “a method for enhancing the field emission efficiency of carbon nanotube emission source”. The film peeling process after the adhesion thereof uses a type to adhere on the surface of the electron emission source. The tape is then peeled off to remove the residue materials described above, thereby achieving the activation purpose. In addition, the recently developed thermal processing of laser or plasma can produce high temperature instantaneously to re-crystallize or remove the non-crystalline carbon. Moreover, using the sandblast processing can also destroy and remove the defective materials.
However, the method as described above still involves disadvantages. For example, since a tape can reach the electron emission source, although the film peeling using a tape is less costly, it is inapplicable to the electron emission source already manufactured in the triode of the cathode structure. In addition, the laser or plasma processing is inapplicable to panels of larger sizes, while the manufacturing cost thereof is very high. Furthermore, the sandblast processing can damage the carbon tube structure, and in applicable to the cathode structure of high pixel resolutions.
Another conventional method is disclosed in the Taiwanese letters patent no. I223308, entitled “a process for enhancing the field emission current of carbon nanotube electron source”. This method uses a thermal glue or a soluble paint to perfuse onto each pixel of the triode structure on the surface of the electron emission source. The thermal glue is peeled off after curing, so as to activate the surface of electron emission source. The advantage of this method is in that the manufacturing process is simple and less costly. However, this method is still very restrictive. One of the restrictions is that the viscosity of the thermally soluble paint is still higher than 1000 cps even during the thermal process. Since the field emission display is of a product of high resolution, the length and width of each pixel is very small. For this reason, air bubbles are easily formed between the thermal glue and the pixel hole even if the high viscosity glue is heated up to a high temperature. The air bubbles will prevent the glue from securely sealing the electron emission source of carbon nanotubes. Accordingly, the surface is unlikely to be activated in full, which will render inhomogeneous activation areas on the surface of the electron emission source, thereby victimizing the display quality.
In addition, the peeling mechanism described above has not been disclosed completely. The peeling process described above requires a homogeneous physical force applied vertical to the electron emission source. If an arbitrary peeling process is performed, a homogeneous activation will not be obtained. In particular, a peeling mechanism applicable to mass production for the future large size displays is especially demanding.